Milk duct openings

Professor Donna Geddes' ground-breaking ultrasound work showed that the face of the lactating nipple has on average nine lactiferous duct orifices (ranging between 4-18). The number of ducts is not linked with capacity to produce or transfer milk.

The circular and radial smooth muscle fibres form a mesh-like structure in the connective tissue around the ducts at the tip of the nipple, with larger fibres found along the ducts as they extend into the nipple stroma.

Connective tissue

The interior of the nipple core is composed of uneven, very dense fibrous or collagen tissue, which supports the soft walls of the lactiferous ducts. Both circular and radial smooth muscle fibres run in this core, directly under the dermis of the nipple, and they contract with stimulation, whether that's by touch or temperature.

Smooth muscle fibres

Contractile radial and circular smooth muscle fibres (which run both in circles around the nipple, and radial, fanning out from the nipple) are found beneath the dermis of the areola, continuous with nipple smooth muscle fibres. Contraction of the nipple and areolar smooth muscle is occurs with involuntary sympathetic nervous system stimulation, in tandem. This contractions results in elevation of the nipple, which has both a visual and tactile effect, orienting your baby for suckling.

Contraction of the smooth muscle of the dermal and nipple stroma capillaries due to sympathetic nervous system adrenergic activation also can result in blanching of the nipple face.

Blood and lymph vessels

Oxytocin not only stimulates contraction of the glandular and ductal myoepithelial cells, it causes expansion (or vasodilation) of blood vessels in the breast. The nipple warms during breastfeeding due to both dilation of its blood vessels and the warmth of the infant’s mouth. This warmth expands the nipple ducts and orifice to help with milk transfer. Nipple duct diameters decrease with cold applications.

Below the areolar muscle there is just a thin layer of fat with blood vessels, which disappears as it approaches the nipple stroma.

There is also a rich lymphatic plexus under the areola, which sits between the skin and the superficial fascia of the breast, and collects the lymph from both the deep and more superficial parts of the breast.

Densely interlaced vessels

Nipple stroma is threaded through with densely interlaced lacteriferous or milk ducts and ductules, small blood vessels, nerves which run to the sensory nerve endings of the dermis, and bundles of smooth muscle tissue. Large nerve bundles are found in the nipple shaft, at the base of the nipple, and also in the glandular tissue adjacent to the nipple-draining ducts.

The milk ducts are lined with cuboidal epithelium, except for the last one of two millimetres of the duct leading up to the orifice, which are lined with the squamous epithelium of the epidermis.

You can see, given how rich the nipple is in blood vessels, milk ducts, and nerves, that bending or deformational forces applied to the interior core of the nipple would cause micro-haemorrhages and breakages, triggering inflammation and pain, which persists in between breastfeeds and also causes stabbing, deep breast pain.

Selected references

Geddes DT. Ultrasound imaging of the lactating breast: methodology and application. International Breastfeeding Journal. 2009;4:doi:10.1186/1746-4358-1184-1184.

Gardner H, Lai CT, Ward LC, Geddes DT. Thermal physiology of the lactating nipple infuences the removal of human milk. Scientific Reports. 2019;9:11854.

Ramsay DT, Kent JC, Hartmann RA, Hartmann PE. Anatomy of the lactating human breast redefined with ultrasound imaging. Journal of Anatomy. 2005;206:525-534.